1. Field of the Invention
The present invention relates to a glass blank employed as a molding material in the press molding of an optical element such as a lens, and more particularly to a glass blank capable of preventing a reaction with the mold member upon press molding, also reducing the adhesive force and the frictional force, thereby avoiding fusion and crack formation in the course of cooling and obtaining a satisfactory optical element, and also to a method for producing an optical element utilizing the glass blank.
2. Related Background Art
For obtaining a satisfactory molded product from the reheating and pressing of glass, it has been a major problem to avoid fusion between the molding glass and the mold member. For this purpose there have been proposed various technologies in the improvement of the mold member, and improvements in the molding material are recently starting to be proposed for preventing the type of fusion mentioned above. Among such proposals for the improvement of the molding material, Japanese Patent Publication Nos. 2-1778, 2-1779, 2-1780 and 61-29890 disclose providing the surface of a glass base with a coating of glass having a higher glass transition point than that of the glass base, a coating of silicon oxide or of carbon. Also, Japanese Patent Laid-open Application No. 1-264937 discloses providing the surface of a glass base with a thin organic layer. Also, the Japanese Patent Laid-open Application No. 62-207728 discloses removing evaporable components on the surface with an acid.
These improvements are effective for avoiding fusion of the glass to the mold member, but the methods disclosed in the above-mentioned references are associated with the following drawbacks:
(a) When the surface of the glass base is provided with a coating of glass having a higher glass transition point than that of the glass base, the glass coating layer is cracked by the molding pressure, and the glass base oozes through the crack causing local clouding on the surface or local fusion of the glass base to the mold member. Also, the adhesive force and the frictional force between the glass blank and the mold member may become large in the course of cooling, eventually causing a crack in the glass base;
(b) When the surface of the glass base is given a coating of silicon oxide, there will be encountered difficulties similar to those in case (a). Particularly, cracks easily occur in the course of cooling, because silicon oxide strongly stickes to with the mold member. The coating tends to crack in the heating stage, because the thermal expansion coefficient of silicon oxide is significantly lower than that of the ordinary optical glass constituting the glass base;
(c) If the surface of the glass base is given an unnecessarily thick carbon coating, the carbon, being a reducing agent, reacts with oxygen in the glass, thus reducing the glass components and giving a brown color thereto. Particularly, if lead-containing glass is used as a glass base, PbO therein is reduced, so that the glass is colored conspicuously and has a lowered transmittance;
(d) When the surface of the glass base is given a thin organic layer, the layer is decomposed upon heating, eventually generating a corrosive gas (such as chlorine or fluorine), thereby contaminating the press molding apparatus and affecting the life of the apparatus including the mold member. Also, the surface precision of the molding apparatus may be deteriorated because the decomposition occurs locally at random;
(e) In some optical glasses, when immersed in acid, not only the volatile components but also the glass itself are dissolved, thus the acid treatment is not usable in such a case.
For avoiding these drawbacks, Japanese Patent Application No. 2-187148 proposes a glass blank having a hydrocarbon coating on glass. According to this proposal, the hydrocarbon coating contains a larger amount of CH.sub.2 in comparison with the carbon coating of a same thickness, thereby being superior in preventing fusion and crack formation, without excessive deterioration of the transmittance of the glass blank.
However, this proposal is still associated with a drawback in that the molded article becomes clouded, though slightly, by the reaction of easily-reactive components in the glass blank, particularly alkali metal oxides and boric, acid, with the hydrocarbon coating.